Connected: An Internet Encyclopedia
RFC 1812 - 3.3.4 Maximum Transmission Unit - MTU

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### 3.3.4 Maximum Transmission Unit - MTU

The MTU of each logical interface MUST be configurable within the range of legal MTUs for the interface.

Many Link Layer protocols define a maximum frame size that may be sent. In such cases, a router MUST NOT allow an MTU to be set which would allow sending of frames larger than those allowed by the Link Layer protocol. However, a router SHOULD be willing to receive a packet as large as the maximum frame size even if that is larger than the MTU.

DISCUSSION
  Note that this is a stricter requirement than imposed on hosts by [INTRO:2], which requires that the MTU of each physical interface be configurable.

  If a network is using an MTU smaller than the maximum frame size for the Link Layer, a router may receive packets larger than the MTU from misconfigured and incompletely initialized hosts. The Robustness Principle indicates that the router should successfully receive these packets if possible.

Next: 3.3.5 Point-to-Point Protocol - PPP

Connected: An Internet Encyclopedia
RFC 1812 - 3.3.4 Maximum Transmission Unit - MTU